Helical or spiral washers are commonly used in pairs to preload threaded fittings on hydraulic actuators. Each washer has a flat surface, a helical surface, and notches along the outer circumference. The helical faces of the washers allow a preload force to be imparted on the threaded components with which they are assembled. Typically, two washers are installed with their helical faces opposing each other. As the washers are rotated in opposite directions against each other, the outer flat surfaces generate a preload force that prevents backlash during through-zero cyclic loading. This preload must be greater than the load generated by the actuator during test cycles to prevent backlash in the load linkage.
Tightening and loosening spiral washers can be labor intensive and time consuming on tests that use many hydraulic actuators. Specially designed spanner wrenches, one per spiral washer, are typically recommended for tightening or loosening the washers. Often the task has been accomplished using impact devices on the spanner wrenches, often resulting in damage to the washer and or the wrench. Such methods do not allow regulation of the amount of preload force applied to the threaded fasteners by the washers.
Although other techniques exist, all prove ineffective on the shop floor due to the magnitude of the forces required, the lack of a method to apply forces simultaneously to each washer, and the lack of a good way to hold one washer “still” while applying wrench forces on the other.